The catalysis of reaction sequences and their integration and regulation in the metabolic pathways of the cell depends on organization at several different levels. These include the organization of protein components in the various organelles and compartments at the cell level, the association of specific protein subunits into enzymes or multienzyme complexes and the organization and association of specific protein domains within the structure of the polypeptide subunits. The long range goal of this research is to define the role of subunit and domain interactions in the structure and function of multifunctional enzyme complexes. By utilizing a specific multi-functional enzyme complex from the tryptophan pathway of Neurospora crassa which is amenable to chemical, physical and genetic analysis we propose to probe domain structure and organization in this multifunctional protein and the role of the interaction of domains and subunits in the expression of enzyme activities.